Polyamide Dimensional Shrinkage Differential under Varied Cavity Pressure Conditions
Cavity pressure directly controls polyamide crystallization and volumetric shrink, requiring precise transducer monitoring to hold DIN 16742 tolerances.
Screw Cushion
Meaning
Remaining material at the front of a reciprocating injection moulding screw after the forward stroke defines the volume of plastic ready for the next cavity fill. This screw cushion acts as the hydraulic buffer that transmits injection pressure to the melt. It ensures that the packing phase effectively compensates for polymer shrinkage during cooling.
When the residue vanishes, the machine loses its ability to control part density and dimensions.
Pressure Stability
The injection stage relies on this held volume to maintain constant force against the cooling melt. Without sufficient residual material, the non return valve fails to seal effectively against backflow. Variations in the cushion size suggest inconsistent feed or irregular melt viscosity across cycles.
Small fluctuations alter the internal stress profile of the final product.
Part Integrity
Consistent dimensions across a production run depend on the precision of this final holding distance. If the distance fluctuates, the weight of the moulded part shifts between shots. Excess material in this zone increases residence time and risks thermal degradation of the polymer.
Heat sensitivity dictates the maximum allowable volume for a specific resin grade.
Control Limit
Operators determine the target distance by observing the point where the screw stops after the hold time expires. An empty zone indicates a short shot where the cavity fails to fill completely. Settings for this parameter are specific to the tool geometry and the flow characteristics of the material.
Stable machine settings prevent the formation of sink marks or voids.